This disclosure relates generally to a switching power supply, and more particularly to power limit detection with primary-side sensing and feedback.
Flyback power converters include a power stage for delivering electrical power from a power source to a load. An example flyback power converter 100 is illustrated in FIG. 1. A switch 102 in the power stage electrically couples or decouples the load 104 to a power source (not shown), and a switch controller 106 coupled to the switch 102 controls the on-time and off-time of the switch 102. The on-time and off-time of the switch 102 may be modified by the controller 106 based upon a feedback signal representing the output power, output voltage, or output current to regulate the same. The energy is stored in the gap of a transformer 108 when the switch 102 is on and is transferred to the load when the switch 102 is off.
The power converter 100 detects output power by measuring the output voltage V_out feeding it back to the primary side controller 106. The output voltage V_out is compared to a reference voltage VREF, and the resulting error signal VC is fed into the controller 106 for controlling the switch 102. Using the output voltage as shown in FIG. 1, the output power P0 is given by:
                              P          0                =                              1            2                    ⁢                      L            M                    ⁢                                    F              sw                        ⁡                          (              EFF              )                                ⁢                                    (                                                KV                  C                                                  R                  S                                            )                        2                                              (        1        )            where LM is the magnetizing inductance of the transformer 108, Fsw is the switching frequency of the switch 102, EFF is the operational efficiency of the power converter 100, and K is a constant. The magnetizing inductance LM and operational efficiency EFF can vary widely between power converters, and VC varies as a function of the load.
FIG. 2 illustrates output power detected by the converter 100 across a range of output voltages. Pout upper bound 202 and Pout lower bound 204 represent, respectively, high and low boundaries for the output power based on the range of possible values of LM, VC, and EFF. In particular, the power limit detection range 208 indicates a range of possible values of the detected output power at a given power limit threshold voltage 206. As shown in FIG. 2A, the power limit detection range 208 of the converter 100 is large, meaning that the precision at which the converter 100 determines the output power is low.
FIG. 3 illustrates a number of design considerations when a power converter has a power limit detection range with a wide variance. A power converter is typically designed to operate within a normal operating range 310, which includes levels of output power below a maximum rated power threshold 311. If the normal operating power range 310 overlaps with the minimum power limit detection threshold 215, the output power detected by the power converter 100 may cause a false power limit trigger. To prevent overlap between the normal operating power range 310 and the power limit detection threshold 215, a small operating margin 312 is designed between the normal operating range 310 and the power limit detection range 208. This increases the difference between the maximum rated power threshold 311 and the maximum power limit detection point 217. However, the power converter 100 may operate above the maximum rated power threshold 311 for short periods of time. Accordingly, to prevent thermal damage to the converter 100, the converter 100 is designed to operate up to the maximum power limit detection point 217. Because the range from the maximum rated power threshold 311 to the maximum power limit detection point 217 is large, the thermal design and operational ratings of components of the power converter are overdesigned to achieve safe operational modes up to the maximum power limit detection point 217.